Investigation of interconnection of communication networks between communication operators has been underway by the GSMA/3GPP (GSM: registered trademark) being a standardization organization of the IP Multimedia Subsystem (IMS). IMS is a technology in a core network for controlling IP packet transport without depending on an access network.
Several technologies related to the IMS have been proposed. For example, PTL 1 describes a technology for allowing a call-ID, a caller telephone number, and a callee telephone number to be output to a log output by a Transition Gateway (TrGW). In a communication system described in PTL 1, when an Interconnection Border Control Function (IBCF) transmits a MEGACO Add request to a TrGW, the aforementioned information is included in the transmission.
On the other hand, an early media specification is defined in 3GPP TS 24.628 Section 4.2.2. The early media is a technology of performing communication between terminals before a call is established.
Several technologies related to IMS or early media in the Session Initiation Protocol (SIP) being a basis of IMS have been proposed. For example, PTL 2 describes a technology for avoiding media clipping in early media when a session request source terminal is compatible with precondition control but a session request destination terminal is not compatible with precondition control. A SIP server described in PTL 2 completes resource reservation for a first terminal being a session start request source. Further, the SIP server completes resource reservation for a second terminal being a session start request destination and transmits a response thereof to the first terminal. Then, the SIP server receives early media from the second terminal and transfers the received early media to the first terminal.
Further, PTL 3 describes a technology for making a Multimedia Ring Back Tone (MRBT) service and a Multimedia Caller identification (MCID) service applicable in IMS. According to the method described in PTL 3, when a caller-side UE is determined to be an MCID service subscriber, and a callee-side UE is determined to support MCID early media, a negotiation between the MCID early media and the callee-side UE is performed, and the MCID is reproduced on the callee-side UE. On the other hand, when a callee-side UE is determined to be an MRBT service subscriber, and a caller-side UE is determined to support the MRBT early media, a negotiation between the MRBT early media and the caller-side UE is performed, and the MRBT is reproduced on the caller-side UE.
Further, PTL 4 describes an early-media service providing method in a SIP infrastructure. According to the method described in PTL 4, an application server receives, from a calling terminal, a first INVITE message including Session Description Protocol (SDP) information of the calling terminal.
The application server operates in a back-to-back user agent (B2BUA) mode and transfers a second INVITE message to a called terminal. Further, the application server acquires, from a media server, media information of a content provided as early media and further sets an early session for conveying the early media from the media server to the calling terminal. The media server provides the early media for the calling terminal through the early session.
When receiving, from the called terminal, a 180 Ringing message including SDP information of the called terminal, the application server prepares for regular session setting between the calling terminal and the called terminal. Additionally, when receiving a 200 OK message from the called terminal, the application server transfers a stop instruction to the media server in order to end the early media transfer. Then, the application server completes the regular session setting between the calling terminal and the called terminal.